Friction stir welding (FSW) is a well known method for joining metallic parts that may be formed of a variety of different alloys. In the FSW process, a FSW pin tool is plunged into the work pieces to be joined until a tool shoulder of the FSW pin is disposed above or is flush with the top surface of the work pieces. A pin portion of the FSW pin is forced into the thicknesses of the pieces to be joined. The FSW pin is driven along a tool path such that friction generated between the FSW pin and the work pieces results in plasticization of an annular region surrounding the pin portion. The resulting joint provides a high-strength, light-weight bond that is producible at relatively low cost and with a reduced process cycle time as compared to other fastening systems.
During the FSW process, it is necessary to prevent movement of the work pieces as the FSW pin is moved along the tool path. In this regard, a fixture assembly may be provided for anchoring the work pieces. The fixture assembly may include a backing anvil against which the work pieces are clamped in the region of the tool path. Due to the extremely high forces required to plunge the FSW pin into the metallic work pieces as well as the high forces required to drive the FSW pin along the tool path, it is necessary that the fixture assembly for securing the work pieces is robust and highly-resistant to movement as any shifting in the position of the work pieces can cause the FSW pin to deviate from the desired tool path. A deviation in the tool path can compromise the integrity of the joint between the work pieces and may require reworking of the joint or scrapping of the work pieces altogether.
Difficulties in securing the work pieces are compounded in cases where the joint between the work pieces has a complex curvature. In such cases, a multi-axis joining machine such as a five-axis FSW apparatus may be employed in order to allow the FSW pin to accurately follow the complex curvature of the desired joint. In addition to regulating the direction of movement of the FSW pin, the five-axis FSW apparatus must also regulate the rotational speed and travel speed of the FSW pin.
Importantly, in order to produce a high quality joint, the work pieces must be clamped against the backing anvil at least in the region of the FSW pin as it moves along the tool path. Furthermore, it is desirable to clamp the abutting surfaces of the work pieces in substantially uniform contact with one another in the region of the FSW pin as it is driven along the tool path. Ideally, uniform pressure is applied across a width of the abutting surfaces of the work pieces to maintain the work pieces in uniform contact with each other and in uniform contact against the backing anvil. Such pressure may be applied by clamping rollers that may be disposed on opposing sides (e.g., on the leading and/or trailing sides) of the FSW pin as it moves along the tool path.
For complex curvature joints, it is necessary that the clamping roller is maintained in a perpendicular orientation of the clamping roller with the backing anvil such that uniform pressure is applied across the width of the abutting surfaces of the work pieces. Failing to apply uniform pressure across the joint may result in a phenomenon known as “backside coining” wherein the finished joint includes raised surface features such as bumps or other surface defects on the backside of the joint (i.e., the side opposite the FSW pin). Removal of these surface features may be necessary and may be facilitated by grinding or sanding of the surfaces after the FSW joining process is complete.
As may be appreciated, the need to rework the joined pieces in order to eliminate backside coining increases manufacturing cost and production time. For certain aerodynamic components, the surface finish tolerances may be relatively small such that even minor defects in the backside surface require reworking to reduce or eliminate the surface features. For example, an engine inlet for an aircraft jet engine may require a surface finish that allows for a substantially laminar flow of air into the jet engine. In this example, surface bumps as small as 0.020 inches must be removed to prevent disruption of the laminar flow.
As can be seen, there exists a need in the art for a system and method for joining parts by FSW wherein a uniform clamping pressure is applied across the work pieces to be joined as the FSW pin moves along the tool path. More specifically, there exists a need in the art for a system and method for joining parts by FSW which provides uniform clamping pressure of the work pieces against the backing anvil in order to prevent the occurrence of backside coining. Even further, there exists a need in the art for a system and method for joining parts by FSW wherein the uniform clamping pressure can be applied as the FSW pin moves along a complexly curved tool path.